The success seems to arise from two things:

1) A fan base who has strongly associated themselves with the show. Cut off from regular new doses of what they love, the comic book becomes the methadone to the TV show’s heroin, if you will.
2) The tie-in story becomes not so much a tie-in to the main story as it becomes the main story itself. Comics tie-ins done during the show are generally not allowed to move the continuity forward in most ways; the pieces must be left where you pick them up. However, something like the current run of Buffy (called Buffy: Year Eight) doesn’t have to be shoe-horned into continuity – it is the continuity. Of course, it helps that the series is overseen and often written by Buffy creator Joss Whedon himself (and, when not written by him, often written by members of the Buffy TV writing team.)

Comics have been thriving on tie-ins to the defunct. We’re seeing recent success with The Muppet Show, The Lone Ranger, Farscape, and more. Of course, it helps that a comic book can be a success with one percent of the audience of a failed network show…